Flat-panel displays are widely used in conjunction with computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a display substrate to display images, graphics, or text. For example, liquid crystal displays (LCDs) employ liquid crystals to block or transmit light from a backlight behind the liquid crystals and organic light-emitting diode (OLED) displays rely on passing current through a layer of organic material that glows in response to the electrical current.
Inorganic light-emitting diodes (iLEDs) are often used as light sources for displays and lighting. Inorganic LEDs are typically made in a semiconductor material (such as GaAs, GaN, or InGaN) that has a large optical index of refraction compared to the glass or polymer substrates on which they are often mounted or the air in which they are viewed. Hence, light is often trapped in the semiconductor material due to total internal reflection and various light management structures are provided in LEDs to improve the light output. When iLEDs are mounted on a glass or polymer substrate, light is also trapped in the substrate because the optical index of refraction of the substrate is less than that of the semiconductor material but more than the optical index of refraction of air. More than 40% or even 50% of the photons formed in a diode junction can be trapped and ultimately lost in such structures. Furthermore, light emitted by one pixel can propagate through a substrate and be emitted near another pixel, reducing the optical sharpness of the display.
It is also important to reduce the reflection of ambient light from a display surface, such as a substrate. In order to improve the display contrast of light-emissive displays, display designers typically use anti-reflection layers on the front cover of displays and light-absorbing layers internal to the display to reduce ambient light reflection. For example, OLED displays often employ circular polarizers on the cover glass and LCDs use an ambient-light-absorbing black matrix in combination with color filters used to color the white light emitted by the LCD backlights. These black-matrix structures are either in a common structure with the color filters or between the viewer and the color filter. For example, U.S. Pat. No. 6,466,281 entitled Integrated black matrix/color filter structure for TFT-LCD describes a light-shielding layer located above the switching transistors in the display. U.S. Patent Application Publication No. 2007/0077349 entitled Patterning OLED Device Electrodes and Optical Material describes a black matrix integrated into an electrically insulating layer to absorb unwanted light in an RGBW configuration. Similarly, U.S. Pat. No. 7,402,951 entitled OLED Device having Improved Contrast discloses a contrast enhancement element with a light-absorbing layer for absorbing ambient light. U.S. Pat. No. 6,812,637, U.S. Pat. No. 7,466,075, and U.S. Pat. No. 7,091,523 all describe the use of black-matrix structures to improve contrast. These light-absorbing elements or layers are located between a viewer and the light-emitting OLED pixels.
Inorganic LED displays are also known to use black-matrix structures, as disclosed in U.S. Pat. No. 7,919,342 entitled Patterned Inorganic LED Device in which a patterned conductive layer between and above the patterned light emitters can act as a black matrix to absorb light and increase the display contrast.
Black matrix structures in conventional displays can locate light-absorbing elements or layers between a viewer and the light-emitting pixels. Although such an arrangement can be relatively effective in absorbing ambient light, they also absorb emitted light and can create viewing-angle dependence for brightness. Such multi-layer structures are more complex and costly to manufacture and the additional layers can also absorb emitted light, reducing display efficiency. Thus, there remains a need for improvements in display systems, pixel structures, and methods of manufacturing that provide improved image quality and contrast, improved emission efficiency, and a reduced manufacturing cost in a mechanically and environmentally robust structure.